1. Field of the technology
The present invention relates to an improvement of a WBS (the white block skipping) method of bandwidth compression of transmitting facsimile signal, wherein picture signal is divided into a specified number of blocks and information to indicate existence of at least a black (or white) picture element therein and information to indicate the black (or white) picture elements.
2. Prior Art
In the conventional facsimile system, when black patterns are printed on a white paper, so called white block skipping method has been used. The white block skipping method is that picture information of one horizontal scanning line is divided into N blocks each including specified number of picture elements, and existence of black pattern in respective blocks are detected, and encoding and transmission are made with respect to the detected signal, and information only of blocks having at least one black pel (picture element) is sequentially transmitted as the picture information.
Such conventional method is elucidated referring to FIG. 1. First, synchronizing signal "a" is issued for a time length of l.sub.1, subsequently a control signal "b" for indicating existence or non-existence of picture information for the block is issued for a time length l.sub.2, and subsequently an information signal "c" only for the block containing picture information, i.e., at least a black picture element is issued. For example, provided that a picture signal in one horizontal scanning line is divided into N blocks wherein N=10, and that only 3rd and 7th blocks contain black picture elements, the control signal "b" has the information that blocks of No. 1, 2, 4, 5, 6, 8, 9 and 10 contain white picture elements only and that blocks of No. 3 and 7 contain black picture elements. The picture information part "c" includes a picture signal for block No. 3 and subsequently that for block No. 7. When signal of a horizontal scanning line consists of white picture element only, then the picture information "c" is not issued at all. By adopting the abovementioned white block skipping method, transmission time length of the facsimile is considerably compressed.
However, the abovementioned white skipping method has a drawback that a time length l.sub.3 of the picture information part "c" varies depending on number of blocks containing black picture elments, while synchronizing signal "a" and control signal "b" have constant time length. In the above-mentioned signal bandwidth compression system, if an error of decoding of the control signal happens, then the time position and number of black picture element in reproduced picture signal part is liable to differ from the contents of the information in the transmitted picture signal. Accordingly, the picture information is liable to be allotted in wrong block or number of blocks of the picture signal becomes excess or deficient resulting in failure of right picture reproduction.